1. Field of the Invention
The present invention relates to a circuit board for use in electronic equipment systems, and more particularly, to a process for manufacturing a ceramic circuit board combined with thin films specifically suitable for electronic computers having a high density packaging of electronic parts.
2. Description of the Related Art
Circuit boards for use in electronic equipment systems such as large scale electronic computers have been intensively required to be capable of interconnecting LSI devices at shorter distances to attain a high processing speed. In order to overcome this difficulty, there has been known a thick film-thin film hybrid circuit board which is produced by laminating green sheets to form a ceramic multi-layer circuit substrate and then forming thin film interconnections on the surfaces thereof by a thin film process, where finer interconnections are formed in a high density by the thin film process to achieve an increased LSI-mounting density with shorter connection distances between the LSI devices. One of the prior processes for manufacturing such ceramic thin film hybrid circuit board comprises forming thin film multi-level interconnections on the surfaces of a ceramic multi-layer substrate with insulating layers composed of primarily organic materials by a green sheet laminating process which can easily make interconnections multi-leveled, as disclosed in Japanese Patent KOKAI (Laid-open) No. 60-148191.
The ceramic multi-layer circuit board as described above has produced problems of the non-homogeneity of conductive materials and ceramics as shaping thereof is performed, the deformation of the substrate caused during firing due to the mismatch in shrinkage between the materials on the order of a warping degree of 0.1 mm/25 mm, and the limitation in accuracy of the thin film patterns produced on the warped substrate by a photolithography technique owing to the variation in depth of the focuses of the patterns projected by an exposing apparatus in the warped substrate. For this reason, the prior art employed the steps of abrading and polishing the surfaces of the ceramic multi-layer substrate to flatness and then forming thin film multi-level interconnections thereon.
Moreover, there is still another problem where the ceramic substrate shrinks when it is sintered causing a tolerance in the degree of shrinkage by each substrate so that after the ceramic substrate is sintered, the connecting pads of the thin film interconnection layer to be formed, can not capture the vias which have been formed on the sintered ceramic substrate. In order to cope with this difficulty, the prior art disposed capture pads having an area capable of covering the variation of shrinkage on the surfaces of the ceramic board.
Furthermore, an attempt has been made to make the roughness of the surfaces of the substrate have less influence on the thin film interconnection layer, which is to be formed on the surfaces of the substrate, by providing thinner capture pads as described above by vapor deposition or sputtering, or plating on the underlayer deposited by the vapor deposition or sputtering.
However, since ceramic particles having a size of several micrometers to 10 .mu.m are used for the aforementioned ceramic multi-layer substrate, in the surface layer of the board obtained after sintering the substrate, there is generated surface roughness owing to sizes of the particles, fine voids formed by the sintering, pin holes and the like, which roughness causes formation of defects in the thin film interconnections to be formed thereon, which defects in turn inhibit formation of much finer thin film pattern. This problem remains to be solved.